What if concrete had been discovered earlier?
Its not a matter of concrete or the lack thereof. Its an immense building project by the standards of the time.
Could a defeat of Salidin have created greater unity between the countries of Catholic Europe? Hence more manpower/troops to build/maintain such a wall?
No. It would be useful for the continued survival of the Kingdom of Jerusalem, and it might make crusades look like a good idea again, but it won't unite Catholic Europe. I'm not sure there's any scenario that can lead to that happening in the Middle Ages for more than a very short while (as in, at most the length of a war or campaign).
And building & maintaining any permanent thing in the Holy Land requires more Catholics (or other loyal subjects) in the Holy Land for the Kingdom of Jerusalem. Unfortunately, a big defeat for Saladin won't lead to that - it might make some difference, but it won't magically make it attractive to settlement.
Besides, it isn't as if the Kingdom doesn't already have castles that can - theoretically, at any rate (which is all the wall can offer) hold and secure the kingdom.